The present invention relates to an exhaust port structure in an internal combustion engine.
There is such a conventionally known exhaust port structure in which a cylindrical exhaust port liner is formed of a ceramic material and formed by casting in a cylinder head body (see Japanese Patent Application Laid-open No. 5-256191, for example).
There is a conventionally known waste heat recovering device for an internal combustion engine utilizing a Rankin cycle, which includes an evaporator for generating vapor having a raised temperature and a raised pressure, namely, raised-temperature/pressure vapor using an exhaust gas from the internal combustion engine as a heat source, an expander for expanding the raised-temperature/pressure vapor to generate an output, a condenser for liquefying vapor having a dropped temperature and a dropped pressure, namely, dropped-temperature/pressure vapor discharged from the expander after the expansion, and a supply pump for supplying a liquid, e.g., water from the condenser to the evaporator. In this case, the waste heat recovery efficiency can be increased by introducing the exhaust gas to the evaporator, while being maintained at a high temperature as much as possible, but the following problem is encountered in the conventional exhaust port structure: substantially the entire outer peripheral surface of the exhaust port liner is in contact with the cylinder head body and for this reason, the heat of the exhaust gas is propagated via the exhaust port liner to the cylinder head body and as a result, the temperature of the exhaust gas is dropped.
It is an object of the present invention to provide an exhaust port structure of the above-described type for an internal combustion engine, wherein the propagation of the heat from the exhaust port liner to the cylinder head body can be inhibited as much as possible.
To achieve the above object, according to the present invention, there is provided an exhaust port structure for an internal combustion engine, wherein a cylinder head has a cylinder head body and a cylindrical exhaust port liner, the cylindrical exhaust port liner being partially supported at a plurality of points on the cylinder head body, and a heat-insulating layer exists around the exhaust port liner.
With the above arrangement, a heat propagation path between the exhaust port liner and the cylinder head body can be reduced remarkably, whereby the propagation of heat from the exhaust port liner to the cylinder head body can be inhibited as much as possible to maintain the exhaust gas at a high temperature.
Sites existing respectively adjacent an inlet and an outlet for the exhaust gas and/or a valve stem-insertion section are selected as a plurality of partially supporting points on the exhaust port liner. By selecting such supporting points, an accuracy of attachment of the exhaust port liner to the cylinder head body can be ensured. In this case, a means for cooling the valve stem-insertion section is provided in the cylinder head body in order to maintain the positional accuracy of the valve stem-insertion section and inhibit the thermal expansion of the valve stem-insertion section to smoothly conduct the opening and closing movements of an exhaust valve.